Medical screws of various designs and material composition are used to affix medical implants, grafts and bone fragments to substrate bone structures during orthopedic surgery. One surgical use involves insertion of an interference screw into a bone tunnel to secure an end of an anterior cruciate ligament (ACL) replacement graft in place. ACL reconstruction procedures and interference screws are disclosed, e.g., in U.S. Pat. Nos. 5,062,843, 4,950,270 and 4,927,421.
Medical screws have typically been fabricated from medically approved metallic materials, such as stainless steel or titanium, which are not absorbed by the body. Screws made of these strong materials exhibit sufficient torsional strength to withstand the torque necessary to insert the screw into bone. A disadvantage of such screws, however, is that once healing is complete, an additional surgical procedure may be required to remove the screw from the patient. Metallic screws may include a threaded shank joined to an enlarged head having a transverse slot or hexagonal socket formed therein to engage, respectively, a similarly configured, single blade or hexagonal rotatable driver for turning the screw into the bone. The enlarged heads on such screws can protrude from the bone tunnel and can cause chronic irritation and inflammation of surrounding body tissue.
Permanent metallic medical screws in movable joints can, in certain instances, cause abrading of ligaments during normal motion of the joint. Metallic screws also occasionally back out after insertion, protruding into surrounding tissue and causing discomfort. Furthermore, permanent metallic screws and fixation devices may shield the bone from beneficial stresses after healing. It has been shown that moderate periodic stress on bone tissue, such as the stress produced by exercise, helps to prevent decalcification of the bone. Under some conditions, the stress shielding which results from the long term use of metal bone fixation devices can lead to osteoporosis.
Biodegradable or bioabsorbable interference screws have been proposed to avoid the necessity of surgical removal after healing. Because the degradation of a biodegradable screw occurs over a period of time, support load is transferred gradually to the bone as it heals. This reduces potential stress shielding effects. Conventional bioabsorbable interference screws commonly have a polymer component and are softer and weaker than metallic screws, such that they are not self-tapping, requiring the hole drilled into the bone to receive the screw to be tapped (threaded). The necessity to tap holes in the injured bone adds to the complexity of the surgical procedure and lengthens the time required to complete the operation.
In addition, screws having a polymer component, hereinafter referred to as “polymer screws” exhibit substantially lower torsional strength than conventional metal screws, making them susceptible to deformation when subjected to the torsional loads required to drive the screw into relatively hard tissue such as bone. The high torque that must be applied to medical screws by a driver can cause shear deformation of the relatively soft polymeric material, causing damage to the screw, e.g., the driver can “strip” the recess or slot provided on the screw for the driver. If the screw is not inserted in bone to the proper depth at the point of the failure, difficulty may arise in driving the screw further in, or backing the screw out.
A number of approaches have been used to alleviate the shear deformation of polymer medical screws including low friction coatings, internal reinforcement with fibers or composite formations, crystalline orientation via subjection to compression and screw head design. In yet another approach, as shown in U.S. Pat. No. 5,169,400 (to Muhling, et al.), U.S. Pat. No. 5,470,334 and EP 0502698A1 (to Ross, et al.), and U.S. Pat. No. 5,695,497 (to Stahelin, et al.), a central cannula or recess having a non-circular cross-section running a portion of or the entire length of the screw is provided. The noncircular cross-section is disclosed as being of various shapes (hexagonal, square, star-shaped, etc. in cross-section, or with a plurality of radial force or lobe members) with a complementarily shaped screwdriver bit to increase torque transfer. Manufacturing tolerances for forming the cannula/bore of the screw and its mating bit limit the amount of surface-to-surface contact between the cannula/bore and bit. Decreased surface-to-surface contact may result in higher stresses and an increased risk of torque failure.
This limitation is not overcome by the approach shown in U.S. Pat. No. 5,584,836 (to Ballintyn, et al.), of using a plurality of cannulae. Multiple cannula and mating driver projections are weaker than a single projection, add manufacturing complexity, and are still subject to manufacturing tolerances. This is especially an issue when complicated geometries are employed. Greater surface-to-surface contact at the driver/screw cannula interface distributes the forces exerted on the screw by the driver, reducing localized stresses and enabling a higher torque to be applied to the driver before the strength limit of the screw material is reached. Efforts to reduce manufacturing tolerances on mating parts can be cost prohibitive and time consuming to both manufacture and inspect.
Accordingly, it would be advantageous to provide a polymeric-based, cannulated medical screw or medical screw with a tool receiving bore and associated driver, wherein the outer surface of the driver and the cannula/bore surface are closely mated to increase the insertion torque tolerance of the screw.